1. Field of the Invention
This invention relates to the art of geophysical prospecting using artificially induced seismic energy, and more particularly to apparatus and methods for generating shear waves suitable for use in seismic exploration methods.
2. Description of the Prior Art
Two types of seismic signals have been used in the seismic exploration of earth strata. One type is the so-called pressure (P) wave in which the earth particle motion is in the same direction as the wave propagation. Pressure waves are sometimes also called compressional or longitudinal waves. The other type is the shear wave in which the earth particle motion is generally normal to the direction of wave propagation. Shear waves in which the particle motion is oriented normal to the incident plane are called horizontal shear (SH) waves and shear waves in which the particle motion is oriented within the incident plane are called vertical shear (SV) waves.
Pressure waves are the most commonly used signals for seismic exploration and may be generated in numerous ways, such as the detonation of an explosive, the dropping of weights or the use of a mechanical vibrator. My U.S. Pat. No. 4,143,747 and my copending application Ser. No. 956,613 disclose a rotating eccentric weight seismic apparatus and method particularly suited to the generation of pressure waves in relatively inaccessible areas.
Recently, however, there has been an increased interest in the use of shear waves in seismic exploration. U.S. Pat. Nos. 3,286,783 to Cherry et al.; 3,302,164 to Waters et al. 3,372,770 to Clynch; 3,835,954 to Layotte and 4,059,820 to Turpening disclose various apparatus and methods for the generation and use of shear waves in seismic exploration. Heiland, C.A., Geophysical Exploration, Prentice-Hall, Inc., 1940, discloses the use of rotating eccentric weight vibrators to generate shear waves.
In the search for petroleum and other valuable resources, it has become the practice to transmit a desired pressure or shear wave signal into the earth from a sourcepoint near the surface of the earth. The reflected and/or refracted energy returning from within the earth to a receiver location is sensed and raw seismic data is recorded. The raw seismic data is mathematically processed and then interpreted to provide an indication of the structure of the underlying strata. In the exploration of onshore regions which are relatively inaccessible to vehicles, the weight of supplies and equipment required determines the practicality of a particular exploration system.
At the present time, a wide variety of seismic exploration systems are available. In some of these systems, a coded energy signal is transmitted into the earth and the raw seismic data which is obtained is correlated with a signature of the coded energy signal. The signature of the coded energy signal must be of very good quality in order to obtain a good quality correlated trace. These coded energy signal systems can be generally classified, according to the method by which the signature used to correlate the raw trace is obtained, as either a master-type or a slave-type source system. In the master-type source systems, the signature used to correlate the raw seismic data is generally sensed as the coded energy signal is transmitted. In the slave-type source systems, the source signature used to correlate the raw seismic data is the predetermined signal which is used to drive the "slave" source during the generation of the coded energy signal.
In the master-type systems disclosed in the prior art, the source signature is typically generated by an acoustic sensor which is responsive to the outgoing seismic signal, such as an accelerometer or a geophone located on or near the seismic wave generator. These master-type systems have generally not been used successfully because the acoustic sensors employed are inherently sensitive to any acoustic energy, including acoustic energy unrelated to the coded energy signal being transmitted. The source signatures generated by these acoustic sensors are often attenuated and/or phase-shifted usually contain significant background interferences. While numerous methods have been proposed to extract the true source code from the output signals generated by acoustic sensors, these methods have been only moderately successful. Accordingly, the quality of the processed traces produced with the prior art master-type coded energy signal systems remains generally unacceptable for high resolution seismic exploration.
On the other hand, the correlated traces obtained by use of the "slave-type" source systems generally have much better resolution. Because the source used in these methods can be made to transmit coded energy signals according to a predetermined code, the code is known and need not be detected by use of an acoustic sensor. Furthermore, carefully preselected coded energy signals which tend to yield high resolution seismic data can be transmitted by precise control of the source. These systems, such as the well known VIBROSEIS.RTM. system developed and licensed by Continental Oil Company, Ponca City, Oklahoma, have been relatively successful. However, the weight of the equipment required, specifically the heavy source control devices and vibrators employed, increases markedly as the resolving power of these systems is enhanced. Moreover, since the VIBROSEIS.RTM. and similar sources must be coupled to the ground, it has been determined that the peak force to weight ratio must be less than 1. These sources are normally vehicle mounted and weigh between about 10 and about 20 tons. Due to this great weight, these "slave-type" source systems are impractical for use in regions not accessible to vehicles.
While the deficiencies of the prior art systems have been largely overcome by the apparatus and method for generating pressure waves disclosed in my U.S. Pat. No. 4,143,737, the apparatus and method disclosed therein are not directly applicable for use in seismic exploration methods using shear waves. Thus, a need exists for a portable apparatus and a method for generating shear waves and obtaining high resolution seismic data with such shear waves.
Accordingly, a primary object of this invention is to provide a portable apparatus and a method for generating shear waves and obtaining high resolution seismic data with such shear waves.
Another object of this invention is to provide an apparatus and method for transmitting coded shear wave signals into the earth and obtaining a code signal for correlation of the resulting seismic data, which code signal is free from the attenuation, phase-shifting and background interferences exemplifying the comparable prior art systems.
Yet another object of this invention is to provide a shear wave generating apparatus and method in which the required weight of the exploration equipment is reduced without sacrificing seismic data quality.
Still another object of this invention is to provide an apparatus and method for simultaneously generating both a multiple impulse pressure wave and a sinusoidal shear wave.
Other objects, advantages and features of this invention will become apparent to those skilled in the art from the following description when taken in conjunction with the drawings.